Linear accelerators (especially those for medical use) accelerate subatomic particles such as electrons to relativistic speeds along an evacuated conduit. Typically, the conduit uses a tuned-cavity waveguide in which a radio-frequency (rf) standing wave is established in order to accelerate the electrons. The electrons arrive at one end of the accelerator from an electron gun such as a thermionic device or a photocathode, and pass through a series of cavities defined within a conductive material and which therefore contain the rf standing wave, one half-phase per cavity. The cavities are sized so that the electrons traverse each cavity in exactly half the time period of the standing wave. Thus, by the time they arrive in the next cavity, which will be in anti-phase with the preceding cavity, the standing wave is at the opposite phase point, i.e. has reversed. The electrons thus always see an electrical field of the same polarity and are accelerated along the length of the accelerator. The dimensions of each cavity are chosen to match the speed of the electrons at that point along the accelerator. They are thus initially shorter in length, becoming longer and more uniform in length as the electrons approach relativistic speeds.
To ensure efficient and smooth operation of the accelerator, it is therefore important that the electrons remain in “bunches” and at the same predictable speed during their passage along the accelerator. If a group of electrons begins to spread out along the longitudinal axis of the accelerator then the foremost or rearmost electrons in the group will being to “see” the wrong phase of the rf standing wave.
One reason why the bunch of electrons might be perturbed is the presence of electrical fields other than the first-order standing wave that is being used to accelerate them. As the accelerating cavities are usually enclosed within a substantial conductive housing, the presence of external stray fields is unlikely, and thus the main source of other fields is higher-order resonances within the cavity. Such modes might impose a longitudinal acceleration on the bunch, affecting its speed and possibly unbunching it, or a lateral force serving to deflect the beam sideways and off the accelerator axis. Thus, it is seen as important to ensure that such higher-order modes (“HOMs”) are inhibited, particularly in commercial or medical accelerators where a steady and uniform beam is relied upon.